1. Field of the Invention
The present invention relates to a grain oriented electromagnetic steel sheet adapted to be used for an iron core of a transformer or other electrical appliances.
2. Description of the Related Art
A grain oriented electromagnetic steel sheet as an iron core material for a transformer, a generator or a motor is required to have a high magnetic flux density and a low-iron loss as the most important properties.
Various measures have so far been taken to achieve a low iron loss of the grain oriented electromagnetic steel sheet. Among others, importance has been attached to high integration of the grain orientations of the steel sheet in the {110} &lt;001&gt; orientation known also as Goss orientation. When grain orientations of the steel sheet are highly integrated in Goss orientation, &lt;001&gt; axes which are axes of easy magnetization of iron crystal would highly be integrated in the rolling direction. That is, force required for magnetization in the rolling direction becomes smaller, resulting in a smaller coercive force. As a result, hysteresis loss becomes smaller, thus permitting achievement of a low iron loss.
Aligning grain orientations in Goss orientation greatly contributes to reduction of noise upon magnetization which is an important required property of a grain oriented electromagnetic material. Magnetostriction vibration and electromagnetic vibration of the iron core material are known to be causes of noise produced from a transformer. An improved degree of integration of grain orientations in Goss orientation inhibits generation of 90.degree. magnetic domain forming a cause of magnetostriction. Simultaneously with this, decreased excited current inhibits electromagnetic vibration, thus resulting in reduction of noise.
For a grain oriented electromagnetic steel sheet, as described above, integration of &lt;001&gt; axes of crystal grains in the rolling direction is the most important subject. As an indicator of the degree of integration, the magnetic flux density, B.sub.8 (T) at a magnetization force of 800 A/m is often employed. That is, development efforts of a grain oriented electromagnetic steel sheet are promoted with improvement of magnetic flux density B.sub.8 as an important target. The iron loss is typically represented by an energy loss, W.sub.17/50 (W/kg) under conditions including an excited magnetic flux density of 1.7 T and an excited frequency of 50 Hz.
The secondary recrystallization grains of the grain oriented electromagnetic steel sheet are formed through a phenomenon known as secondary recrystallization during the final finishing annealing. Enormous growth of crystal grains in Goss orientation is selectively caused by secondary recrystallization to increase the degree of integration in Goss orientation, thus obtaining a product having a desired magnetic property. In order to effectively accelerate integration of secondary recrystallization grains in Goss orientation, it is important to form a precipitation dispersion called an inhibitor which inhibits normal growth of primary recrystallization grains, uniformly throughout the steel and in an appropriate size. Presence of the inhibitor makes it possible to inhibit normal grain growth of primary recrystallization grains, and maintain a fine state of primary recrystallization grains even at high temperatures during final finishing annealing. At the same time, there is provided a higher selectivity for the growth of crystal grains in a preferred orientation, thus resulting in a higher degree of integration of crystal grains in Goss orientation and permitting achievement of a high magnetic flux density. In general, it is believed that a higher degree of integration in Goss orientation is available when the inhibitor is stronger and the normal growth inhibiting ability is great.
A material having a small solubility in steel such as MnS, MnSe, Cu.sub.2-x S, Cu.sub.2-x Se or AlN is applicable as an inhibitor. For example, Japanese Patent Publication No. 33-4710 and Japanese Patent Publication No. 40-15644 disclose adding aluminum to a material, using a high reduction within a range of from 81 to 95% for the final cold rolling, and applying annealing before the final cold rolling, thereby causing precipitation of AlN, a strong inhibitor.
Further, it is known that, in addition to the inhibitor constituents mentioned above, addition of Sn, As, Bi, Sb, B, Pb, Mo, Te, V, or Ge is effective for improvement of the degree of orientation integration of secondary recrystallization grains.
From among these additional inhibitor constituents, P, As, Sb and Bi falling under the category of 5B family elements in the Periodic Table are known to intensify the normal grain growth inhibiting ability and improve magnetic property is cooperation with the main inhibitor such as MnS, MnSe, Cu.sub.2-x S, Cu.sub.2-x Se or AlN through segregation on grain boundaries. Among others, bismuth is considered helpful as a component intensifying the normal grain growth inhibiting ability through a grain boundary segregation effect because of a particularly low solubility in iron.
A technique to improve magnetic property by adding bismuth is disclosed in Japanese Examined Patent Publication No. 51-29496 and Japanese Patent Examined Publication No. 54-32412. Japanese Patent Publication No. 62-56924, Japanese Unexamined Patent Publication No. 2-813673 and Japanese Examined Patent Publication No. 7-62176 disclose methods of compositely adding AlN, MnSe or MnS together with bismuth into steel. These techniques, while utilizing the inhibiting power intensifying effect by bismuth, have not as yet been established manufacturing conditions appropriate for a material added with bismuth, and are therefore insufficient to obtain stably a grain oriented electromagnetic steel sheet having satisfactory magnetic property.
Japanese Unexamined Patent Publications Nos. 6-88171, 6-88172, 6-88173 and 6-88174 disclose the possibility of largely improving magnetic flux density by adding bismuth to an aluminum-based inhibitor. The effect itself of addition of bismuth has however been known, but the magnetic property improving effect has not as yet been stably derived.
A method of stabilizing magnetic property of an electromagnetic steel sheet containing added bismuth is disclosed in Japanese Unexamined Patent Publication No. 6-158169. This publication, while mainly disclosing a technique of heating a steel slab having a low sulfur or selenium content to a low temperature and performing nitriding during heating, discloses also a manufacturing method comprising the steps of adding bismuth to steel and carrying out the latter half of decarburization annealing in a reducing atmosphere. However, the decarburization annealing conditions in this techniques mainly aims at stabilizing formation of a film. That is, optimum conditions for stabilizing the magnetic property improving effect for a material added with bismuth have not as yet been established.
Regarding a separator for final finishing annealing, Japanese Unexamined Patent Publication No. 8-253819 discloses a technique of forming a film having an amount of coating of at least 5 g/m.sup.2 per side of the steel sheet. This technique has an object to improve the film through improvement of gas ventilation between coil layers, not providing a function of stabilizing magnetic property. Further, according to the result of research conducted by the present inventors, a simple increase in the amount of coated separator would result in a reverse effect for the stabilization of the magnetic property.
As to the technique of using a low-activity material as an annealing separator for the silicon steel with added bismuth, Japanese Unexamined Patent Publication No. 6-256849 discloses a method of coating a material low in reactivity with SiO.sub.2 after application of a nitriding treatment. However, the function of bismuth in this technique is only to prevent decomposition of the inhibitor during a final finishing annealing unique to a mirror-finishing material including a nitriding step. Japanese Unexamined Patent Publication No. 7-173544 discloses a manufacturing method of a mirror-finished grain oriented electromagnetic steel sheet by coating an annealing separator added with a metal chloride onto a silicon steel with added bismuth. This technique has as well a main object to obtain a mirror surface by the addition of bismuth into the steel, and consequently, a satisfactory magnetic property cannot stably be obtained unless decarburization annealing conditions are controlled.
Japanese Unexamined Patent Publication No. 9-202924 discloses a method of coating alumina as an annealing separator after carrying out decarburization annealing in an atmosphere not generating iron oxides, or removing oxides from the surface of the decarburization-annealed sheet. In this technique, alumina is used as an annealing separator for the purpose of obtaining a satisfactory magnetic property without being affected by the gas ventilation between coil layers during final finishing annealing. Application of this technique permits achievement of reduction of the amount of oxygen on the surface of the final-finishing-annealed sheet under the effect of the alumina separator, and stabilizes the magnetic property to some extent. However, since the decarburization annealing conditions are favorable only for mirror surface finishing, secondary recrystallization grains cannot be completely stabilized. When using alumina as an annealing separator, it becomes difficult to remove impurities from the steel, and brings about a problem of deterioration of hysteresis loss.
In other words, addition of bismuth, being very helpful for the improvement of the magnetic property of a grain oriented electromagnetic steel sheet, tends to cause defective secondary recrystallization under the effect of various factors, and leaves a difficulty in stably obtaining a satisfactory magnetic property.
The present invention has, as an object, to stabilize secondary recrystallization of a grain oriented electromagnetic steel sheet with added bismuth, and permit manufacture of a grain oriented electromagnetic steel sheet having excellent magnetic flux density and iron loss.